The term “oscillatable unit” is intended to mean a unit which produces and/or receives mechanical oscillations, with the oscillations depending on certain variables predetermined by the unit itself. Such oscillatable units are known, for example, in the form of oscillatory rods, or oscillatory forks, for fill-level limit-detection or in the form of an oscillatory membrane of an ultrasonic transducer, or in the form of a measuring tube of a measurement pickup of vibration-type inserted into the course of a pipeline. Measurement pickups of vibration-type are used, for example, for measuring the mass flow, a density and/or a viscosity of a fluid flowing in a pipeline. In the case of such oscillatable units, the frequency, or wavelength, depends e.g. on inertia and stiffness, when the unit is an oscillatory rod, or on the predetermined velocity of sound in the unit and is likewise influenced by stiffness, when the unit is an ultrasonic transducer. A problem of such oscillatable units is that they must be tuned to certain frequencies, or wavelengths, in order to be usable for certain media or in order to be able to make use of certain advantageous frequency ranges. Furthermore, also most often the corresponding evaluation units, with reference to the used electronics, are designed for particular frequency ranges. In the face of this, the fact remains that the process conditions existing in the application have effects on the tuned resonance frequency, so that such also can be influenced or changed. In the case of so called single-rods (and also, in general, in the case of sensors which come in direct contact with the medium to be measured), the material can e.g. acquire accretions or experience corrosion. Both of these change the mass, and, consequently, also the resonance frequency of the single-rod. In the case of ultrasonic sensors, the resonance characteristics are changed e.g. by the process temperature. Thus, there are process conditions which can nullify the calibration or tuning to the desired frequency.